Porosity and grain structure have long been recognized as factors affecting mechanical properties, especially fatigue performance, of cast components. Porosity forms due to volume shrinkage from liquid to solid during solidification, while undesirable grain structures (including large and columnar grain formations) can form if solidification temperatures are not carefully controlled. These problems are particularly acute in the casting of lightweight metal alloys (such as aluminum-based alloys in general and the Al—Si alloys (319, 356, 390 or the like in particular) that are used to make—among other things—automotive cylinder blocks and heads.
Regarding porosity, the evolution of dissolved gases as a result of the significant decrease in solubility of the gases in the solid as compared to the liquid metal is often the primary cause. This is especially true for aluminum-based castings, where hydrogen-induced porosity is the dominant form due to hydrogen being the only gas that is appreciably soluble in molten aluminum. As such, there are several methods that are currently employed to reduce inclusion and hydrogen content in liquid aluminum. These methods include various degassing techniques, including rotary impeller degassing, tablet (such as hexachloroethane (C2Cl6)) degassing, vacuum degassing and spray degassing. Although such degassing methods have demonstrated effectiveness to varying degrees in refining aluminum-based melts, they can cause environmental problems (for example, due to Cl2 gas release) or involve significant capital investment.
Regarding grain structure, it is desirable to pursue fine and equiaxed grain structure in aluminum-based castings as a way to minimize shrinkage, hot tearing and fatigue susceptibility, as well as giving a more uniform distribution of fine scale second phases and microporosity. These in turn improve yield strength, fracture toughness and other useful mechanical properties. Generally, any factor which increases the number of nucleation sites or reduces growth rate has a tendency to yield fine grains in an as-cast aluminum alloy. Commonly-used techniques include using a chill or related insert in the mold to increase local solidification rate (which in turn tends to promote grain size reduction and related mechanical properties). For instance, in a sand-cast engine block, the bulkheads near the crankshaft journal areas are formed with heavy metal chills to assure the required mechanical properties. Unfortunately, when chills are used, undesirable local columnar grain structure may form; such structure can significantly reduce the fatigue performance of the material. Therefore, in practice grain refiners in the form of chemical or elemental additives (such as Ti, B, C or combinations thereof) are often placed in the liquid metal or mold prior to mold fill when a chill is employed. Because the addition of such a grain refiner to a liquid metal melt in the furnace tends to lead to sludge settling over time, such an approach can significantly contribute to furnace and recirculation pump maintenance costs. Likewise, in-mold grain refinement tends to produce more oxides (which can contribute to undesirable bi-film formation) and microstructure segregation in the casting. As such, the present inventors believe that both of these approaches to grain refining should be avoided.